1. Field of the Invention
The present invention relates to an inverter circuit for converting dc electric power to ac electric power, which is to be supplied to a motor for a steering power assisting system.
2. Description of the Related Art
Generally, an inverter circuit includes an inverter constituted of a plurality of series-connected pairs of a high-side switching element and a low-side switching element and a PWM (pulse width modulation) control circuit. The inverter is controlled by the PWM control circuit to convert dc power into ac power as disclosed in JP-A 2004-201414. If each of the high-side and low-side switching elements is a MOSFET, the drain of the low-side switching element is grounded, the source of the positive side MOSFET is connected to a battery terminal, and the gate of each MOSFET is connected to the PWM control circuit.
In order to prevent short circuiting of the inverter from the battery terminal to the ground, it is necessary to provide a time lag or a dead time in the switching operation between the high-side switching elements and the low-side switching elements. Therefore, the inverter circuit can not provide its output current during the dead time even when the PWM control circuit sends a command voltage signal to the inverter circuit. As a result, the output ac power of the inverter circuit includes a waveform distortion, which may cause torque ripples when the output ac power is supplied to an ac motor. In JP-A 2004-201414, the waveform distortion is compensated or corrected by calculating a voltage deviation based on the command voltage signal, an output ac voltage of the inverter and an input dc voltage. However, the above way of compensation can not work very well if the dc power voltage changes.
The inventors conducted various tests and found that a dead zone of operation of an inverter, which is caused by the dead time, changes as the input dc power source voltage changes.